1. Field of the Invention
The invention relates to a negative-working radiation-sensitive mixture containing
a) a compound which generates a strong acid under the action of actinic radiation, PA1 b) a compound having at least two groups crosslinkable by means of acid and PA1 c) a polymeric binder which is insoluble in water and soluble or at least swellable in aqueous alkaline solutions.
The invention also relates to a radiation-sensitive recording material produced with this mixture which is suitable for producing photoresists, electronic components, printing plates or for chemical milling.
2. Description of Related Art
The continuing reduction in the size of the structures, for example in chip manufacture down into the range of less than 1 .mu.m, requires modified lithographic techniques. To form images of such fine structures, radiation of a short wavelength is used, such as high-energy UV light, electron beams and X-rays. The radiation-sensitive mixture must be adapted to the shortwave radiation. A compilation of the requirements to be met by the radiation-sensitive mixture is given in the article by C. G. Willson "Organic Resist Materials--Theory and Chemistry" [Introduction to Microlithography, Theory, Materials, and Processing, editors L. F. Thompson, C. G. Willson, M. J. Bowden, ACS Symp Ser , 219, 87 (1983), American Chemical Society, Washington].
There is therefore an increased demand for radiation-sensitive mixtures which can be used in the more recent technologies, such as mid-UV or deep-UV lithography, exposure, for example, with excimer lasers at wavelengths of 305 nm (XeF), 248 nm (KrF), 193 nm (ArF), electron beam lithography or X-ray lithography, and which, furthermore, are preferably sensitive in a wide spectral region and correspondingly can also be used in conventional UV lithography.
Negative-working radiation-sensitive mixtures which contain bisazides as crosslinking agents and binders derived from isoprene are known. They are used as radiation-sensitive layers in the production of printing plates, printed circuits and integrated circuits. Their use in microlithography is, however, restricted by various technical disadvantages. Particularly, it is difficult to produce qualitatively high-grade layers without pinholes. The heat holdout of such mixtures is inadequate, i.e., the resist images are distorted by thermal flow during processing. Finally, their resolution capacity is restricted to structures of &gt;2.mu.m. During the necessary development with organic solvents, they show undesirably high swelling even in the hardened areas, which in turn causes structure distortions or inhomogeneous developing processes and hence inadequate reproduction of the image predetermined by the exposure mask.
To be able to produce resist images having a resolution of better than 2.mu.m, other negative-working radiation-sensitive mixtures have been developed which are sensitive to radiation of shorter wavelength, for example, to high-energy UV radiation, electron beams or X-rays. Such a mixture contains, for example, a copolymer of 2,3-epoxypropyl methacrylate and 2,3-dichloropropyl methacrylate (DCOPA) or a combination of the corresponding homopolymers. The glass transition temperature of this mixture is, however, too low for many applications and, in particular, the low resistance of the mixture to plasma etching is to be criticized. Furthermore, even this resist material must be processed using developers based on organic solvents which are rather prone to pollute the environment. However, other hitherto known negative-working, aliphatically based photoresists also show a low resistance to plasma etching.
In European Patent Application 0,164,248, an acidcurable mixture is described which can be developed in aqueous alkaline media, has an improved plasma-etching resistance due to the use of aromatics and is sensitive to near UV light (350 to 450 nm). The acid generators mentioned here are especially sulfonic acid ester derivatives of diazonaphthoquinone, which form weakly acidic carboxylic acids on exposure and are therefore effective only in a comparatively high concentration. Due to the weak absorption and the inadequate bleaching properties of the photolytic acid generator, however, such mixtures have a low sensitivity to DUV radiation, electron beams and X-rays.
In U.S. Pat. No. 3,692,560, an acid-curable mixture is described which contains an acid-crosslinkable melamine derivative, a novolak and chlorinated benzophenones as photolytic acid generators. These mixtures again do not have an adequate sensitivity in the deep UV region. Furthermore, hydrohalic acids are undesired as crosslinking catalysts, since these can, during the subsequent doping processes, undergo reactions with the dopants. Moreover, hydrohalic acids remaining in the cured resist have a strong corrosive action and can cause destruction of the material to be imaged and of the production equipment.
The same applies to the acid-generating derivatives of DDT, mentioned in European Patent Application No. 0,232,972, which are highly toxic and, if only for this reason, cannot be suitable for practice. Nevertheless, such compounds show a considerable sensitivity in the deep UV region (200 to 300 nm).
As the compounds which generate a strong acid on irradiation, especially onium salts, such as diazonium, phosphonium, sulfonium and iodonium salts of nonnucleophilic acids such as HSbF.sub.6, HAsF.sub.6 or HPF.sub.6 as described in J. V. Crivello, Polym. Eng. Sci., 23 (1983) 953, have hitherto been used. In addition, halogen compounds, especially trichloromethyltriazine derivatives or trichloromethyloxadiazole derivatives, o-quinonediazidesulfonyl chlorides, o-quinonediazide-4-sulfonic acid esters, organometal/organohalogen combinations, bis(sulfonyl)diazomethanes, sulfonylcarbonyldiazomethanes (See DE-A 3,930,087) or nitrobenzyl tosylates [See F. M. Houlihan et al., SPIE Proc., Adv. in Resist Techn. and Proc. 920 (1988) 67] have been recommended.
These compounds are used in negative- or positive-working radiation-sensitive mixtures. The use of such photolytic acid generators involves, however, also certain disadvantages which drastically restrict the possible uses thereof in various fields of application. Thus, for example, many of the onium salts are toxic. Their solubility is inadequate in many solvents, which is why only a few solvents are suitable for preparing a coating solution. Furthermore, when the onium salts are used, undesired foreign atoms are sometimes introduced which can cause interference with the process, especially in microlithography. Moreover, the onium salts form Broenstedt acids, which have a very severe corrosive action, in the photolysis. These acids attack sensitive substrates, so that the use of such mixtures leads to unsatisfactory results. The halogen compounds and also the quinonediazidesulfonic acid chlorides also form hydrohalic acids which have a severely corrosive action. In addition, since such compounds also have only a limited storage life on certain substrates, it was attempted to improve this by inserting an interlayer between the substrate and the radiation-sensitive layer containing photolytic acid generators. However, this measure led to an undesired increase in defects and to diminished reproducibility (DE-A 3,621,376 U.S. Patent No. 4,840,867).
In more recent papers by F. M. Houlihan et al., SPIE 920, 67 (1988), it was shown by reference to positive-working systems that, in addition to the above-mentioned acid generators, nitrobenzyl tosylates, which on exposure also generate sulfonic acids having a low migration tendency, can be used in certain acid-unstable resist formulations. It can be deduced from these results that such compounds can also be used for photo-curable systems. The sensitivities thus achieved and the thermal stability of the photoresists proved, however, to be inadequate.
In spite of the intensive research activity so far carried out in this field, no radiation-sensitive mixture is at present known, by means of which a negative-working radiation-sensitive recording material can be produced which has a high sensitivity in the DUV region, that is 200 to 300 nm, and high resolution, and which, on irradiation, does not release an acid which has a corrosive action, and can be developed in aqueous alkaline media.
There is therefore a demand for radiation-sensitive mixtures which contain photolytically acting acid generators and which do not have the disadvantages described above and thus possess an adequate reactivity and acid strength for causing compounds of type (b) to crosslink even in short exposure times.